When we discuss the six internal sensory faculties (cakkhu, through mano), they are INDRIYA when no “san” (lobha, dosa, moha) involved. That involves only “phassa.”
– When sensory faculties are used with a defiled mind, they become AYATANA. That involves “samphassa” or “san” + “phassa.” That rhymes as “samphassa.”
That is the easiest way to remember. But INDRIYA can be many types in other contexts.
Regarding the Vipaka PS:
chaṭṭhāyatana is the sixth ayatana or manayatana.